1. Field of the Invention
The present invention relates to a polyvinyl pyrrole host material, a luminescent layer including the same, and an organic electroluminescent display device including the luminescent layer, and more particularly, to a polyvinyl pyrrole host material which has improved energy transfer, a luminescent layer including the same, and an organic electroluminescent display device including the luminescent layer with improved efficiency.
2. Description of the Related Art
An organic electroluminescent display device is a self-emissive display device using the principle that when a current is applied to a fluorescent or phosphorescent organic compound thin layer, electrons and holes combine in an organic layer and thus light is generated. The organic electroluminescent display device is lightweight and has simple elements, and the manufacturing process thereof is straightforward, providing high quality images and a wide viewing angle. Also, the organic electroluminescent display device can provide high color purity and realize perfect mobile images and is operated at a low power and a low driving voltage, which are appropriate properties for mobile electronic equipment.
Generally, an organic electroluminescent display device includes an anode formed on a substrate, and a hole transporting layer, a luminescent layer, an electron transporting layer, and a cathode are sequentially formed on the anode. The hole transporting layer, the luminescent layer, and the electron transporting layer are organic layers formed of organic compounds. The above described organic electroluminescent display device operates as follows. When a voltage is applied between the anode and the cathode, holes injected from the anode migrate via the hole transporting layer to the luminescent layer. The electrons are injected from the cathode via the electron transporting layer to the luminescent layer, and carriers are recombined in the area of the luminescent layer to generate excitons. The excitons radioactively decay and thus light at a wavelength corresponding to a band gap of the material is emitted.
The material forming the luminescent layer of the organic electroluminescent display device can be classified into a fluorescent material using singlet excitons and a phosphorescent material using triplet excitons according to the light emitting mechanism. Such fluorescent materials or phosphorescent materials are used by themselves or doped in an appropriate host material to form a luminescent layer, and as a result of electron excitation, singlet excitons and triplet excitons are formed in a host. The statistical ratio of the singlet excitons and the triplet excitons is 1:3 (Baldo, et al., Phys. Rev. B, 1999, 60, 14422).
When the luminescent layer of the organic electroluminescent display device is formed of a fluorescent material, triplet excitons generated in the host are wasted, whereas when the luminescent layer is formed of phosphorescent material, both singlet and triplet excitons can be used and thus the internal quantum efficiency may reach 100%. Accordingly, when formed of a phosphorescent material, the luminescent layer can have much higher luminous efficiency than when formed of a fluorescent material.
Also, the material forming the luminescent layer can be classified into a luminescent material used alone and a light emitting material employing a host/dopant light emitting system. Recently, M. E. Thompson reported in Chem. Mat. 16, 4743 (2004) an excellent result of a derivative including silicon and having a very wide gap energy that is used as a host for a blue light emitting material compared to a light emitting material used alone.
The light emitting material used alone has problems such as forming excimers by interaction between molecules and reduction in color purity and efficiency. The host/dopant light emitting system has improved color purity compared to the light emitting material used alone and can achieve an increase in efficiency by energy transfer. However, a new host material with improved energy transfer is still required to manufacture a high efficiency light emitting device.